Between five and 10 times a day, an arc flash explosion occurs in electric equipment in the United States that sends a burn victim to a special burn center, according to statistics compiled by CapSchell, Inc., a Chicago-based research and consulting firm that specializes in preventing workplace injuries and deaths.
In response to these statistics and the obvious detrimental affects of arc fault incidents on workers, in the United States the Occupational Safety and Health Administration (OSHA) has begun enforcing recommendations by the National Electric Code (NEC) and National Fire Protection Association (NFPA) regarding employee safety procedures when work on energized systems must be performed. Admittedly, it is preferable and mandated that, when possible and practical, electrical systems are to be worked on in a zero energy state (ZES). However, this condition does not exist under all circumstances and sometimes work on energized systems is necessary.
The 2000 release of the NFPA's 70E document, incorporated herein by reference, recommended the use of personal protective equipment (PPE) based on the potential for exposure to heat energy radiated by electric arcs. NFPA 70E specifies the need for proper personal protective equipment (PPE), in all conditions where there is a possibility of harm induced due to electrical arcing. Previously, electric shock was thought to be the primary and most frequent type of injury sustained when working with electrical systems. A recent study conducted by the National Institute for Occupational Safety and Health on injuries sustained during work with electrical components points to the fact that a significant portion, approximately 40 percent, of these injuries were due to arc flash; an arc flash exposure may result in severe burns to the skin and, in some cases, death.
Burns are sustained due to exposure to a heat source, in this case the heat radiated from an electrical arc. Arcs have temperatures of around 35,000 degrees F. (19500° C.). Distance plays a role in the degree to which injury is sustained. The amount of energy absorbed by the skin at any given time is a function of the temperature of the heat source and the distance from this source to exposed skin. In this case, incident energy is typically calculated in cal/cm2. An energy density of 1.2 cal/cm2 is sufficient exposure to result in second degree burns on exposed human skin.
In 2002, the NEC 70-2002 document, incorporated herein by reference, further expanded on this requirement by mandating that all electrical services that can be accessed while energized be labeled with the hazard category as defined by the NFPA. However, neither document has yet to specify the method by which these values are to be calculated. In response to this fact, the Institute of Electrical and Electronic Engineers (IEEE), in 2004, issued Standard 1584, fully incorporated herein by reference and made a part hereof. IEEE 1584 gives the electric power industry a way to gauge arc-flash hazards. It lets designers and facility operators determine arc-flash hazard distance and how much incident energy employees might be exposed to when they work on or near electrical equipment. These calculations form the basis for re-engineering systems to reduce incident energy to manageable levels or to provide guidance for the appropriate level of personal protective equipment (PPE) to be worn while working on or near energized equipment.
As stated previously, the NFPA 70E document requires calculation of arc fault incident energy but neither provides or specifies any one method of determining this value. Several methods of determining arc fault incident energy have been proposed and are acceptable methods, as defined by the NFPA. These methods include the IEEE 1584, NFPA 70E, Lee's Calculation, ARCPRO by Kinetrics of Toronto, and the Duke Heat Flux Calculator, by Duke Energy. The IEEE Standard, Duke Heat Flux, and NFPA 70E use equations developed from empirical testing, while the Lee paper and ARCPRO use equations based on theoretical analysis.
Article 130 of the NFPA 70E document, incorporated herein by reference, details the requirements for establishment of boundaries for safe working under live circuit conditions.
The IEEE 1584 standard is only one of several methods of calculating potential arc fault incident energy, but is widely used in the industry. The variables used in the IEEE calculations can be readily obtained with some knowledge of enclosure geometry, wire spacing, and fault duty; also, the IEEE standard has been tested and validated for a wide range of conditions. It specifies that the Lee equations should be used for voltages above 15 kV. The calculations consider three-phase arcs in enclosures and in air. The standard is applicable for input ranges for voltage of 208 to 15,000 volts, bolted fault current of 700 A to 106 kA, equipment enclosures of commonly available sizes, and gaps between conductors of 13 mm to 152 mm (0.5 to 6 inches). The equations were developed from curve fitting of results of values measured from testing performed by the standard's working group. Several general conclusions resulting from their testing were found. System X/R (reactance/resistance) ratio, system frequency, and electrode material had little or no effect. Instead, the incident energy depends primarily on arc current. The buss gap (arc length) is only a small factor in the final result.
The IEEE 1584 standard outlines nine procedural steps in determining arc fault incident energy:
1. Collect the system and installation data
2. Determine the system modes of operation
3. Determine the bolted fault currents
4. Determine the arc fault currents
5. Find the protective device characteristics and duration of the arcs
6. Document the system voltages and classes of equipment
7. Select the working distances
8. Determine the incident energy for all equipment
9. Determine the flash-protection boundary for all equipment
The document states that by far, the majority of the work in completing an arc flash assessment is in the collection of system and installation data (step 1). This singular step is expected to account for fully one-half of all the effort in performing such a study. Obtaining the fault duty at a particular electric node can be difficult to determine. Often, wiring diagrams for electrical installations are outdated or lacking necessary information, such as wire size or feeder length, or the drawings may be incorrect all together. Rotating loads and varying generation, particularly near the node of interest can also have major effects on fault duty and is time varying in nature. The majority of remaining analytical work is contained in steps 2 and 3.
Many entities are currently not compliant with OSHA regulations concerning arc-flash assessments and documentation. Previously, OSHA has taken a lenient stand on this issue because methods for determining boundaries as defined the NFPA have only recently been developed. However, now that the IEEE 1584 standard has been accepted as a viable method of performing these assessments, OSHA has begun vigorously enforcing these requirements to better protect workers from this hazard.
By effectively skipping steps one and two of the nine step procedure outlined by the IEEE, a significant source of manpower, time and money can be eliminated from an arc flash assessment. The development of a method to reduce the effort required to reach these end results could radically impact the compliance issues now being faced by most industrial and commercial customers.
OSHA requires all electrical panels under its jurisdiction to be labeled to indicate the appropriate amount of PPE required while working inside of the panel with it energized. Currently, OSHA code requires that all employers make an effort to investigate the potential for injury due to arc flash; noncompliance with this directive may result in monetary penalties and liability in the event of an accident. This new requirement has prompted an influx in the awareness of the potential damage as a result of arc flash and consequently is forcing engineers to find ways to determine the appropriate level of protection required in each case. Companies may spend millions of dollars on arc flash assessment surveys and currently only a very limited number of entities are providing these assessments because of the high cost in manpower and time.
There exist few methods for establishing the potential for exposure to arc flash energy that are currently available. The classical method of obtaining fault current is to determine fault current capacity from information based on power system information.
      I          Fault      -      duty        =            V      Nominal              Z              Service        -        Impedance            
Typically the electric utility company that supplies a site with electrical power can give service impedance based on fault duty calculations and system models. IEEE 1584 specifies that “available fault data must be realistic; not conservatively high.” The document goes further to offer the following reasons for this requirement:                “Available bolted fault currents should be determined at the point of each potential fault. Do not use overly conservative bolted fault current values. A conservatively high value may result in lower calculated incident energy than may actually be possible depending on the protective device's time-current curves. The lower results would be caused by using a faster time-current response value from the protective device's time-current curve.”        
Overestimating fault current can be dangerous for the simple reason that protective devices often have an inverse or extremely inverse time curve. This means that the length of time before operation is inversely proportional to the amount of current flowing through the fuse. For example, using a fuse with a current rating of 200 A and an arc of 1 kA would blow in 0.5 seconds whereas an arc of 500 A would blow in 9 seconds. The first condition results in a delivery of 500 A-s (coulombs) of electric charge and the second results in an exposure of 4500 A-s of electric charge. Therefore, the likelihood of significant bodily injury due to incident energy exposure may be greater under the lower arc fault condition. Following this reasoning, conservatively high arc fault duty estimations may result in underestimating the potential for exposure to incident energy if protection curves are taken into account; protection data is one of the parameters taken into account in the IEEE 1584 calculations.
Another method of performing arc fault assessment include a detailed analysis of the system in conjunction with specialized computer software to simulate and make determinations of arc flash incident energy potential. To adequately complete such an analysis, an effective power system impedance is once again assumed at the service to the site of interest. This type of analysis goes a step further to include all wires sizes and lengths, protection equipment, and enclosure types for the system. This data can then be entered into the appropriate analysis program and results obtained. While this method is valid and has a high degree of accuracy, it tends to be time and labor intensive, leading to large costs for compliance. Also, in many cases, one-line diagrams for an industrial site are out-dated and do not contain modifications that have been completed over the years. Incorrect drawings may result in incorrect estimates on arc-fault potential due to errors in fault duty.
One recent paper has been presented in the IEEE Transactions on Power Systems that suggests an alternate method of obtaining bolted arc fault potential. “Using a Microprocessor-Based Instrument to Predict the Incident Energy From Arc-Flash Hazards” by Baldwin, Hittel, Saunders, and Renovich, suggests the application of current injection at varying frequencies to obtain impedance values; frequency modulation of the current signal quite accurately yields an X/R ratio and application of Ohm's Law will yield the impedance modulus. To obtain accurate results, current levels in the 30 amp range are specified to mitigate the effects of power system noise. This implies that a significant amount of power may be required, particularly on higher voltage systems.
Therefore, what is needed is a device and method that overcomes many of the challenges in the art, some of which are presented above.